BULLETIN DE L ’INSTITUT ROYAL DES SCIENCES NATURELLES DE BELGIQUE SCIENCES DE LA TERRE, 74-SUPPL.: 73-80, 2004BULLETIN VAN HET KONINKLIJK BELGISCH INSTITUUT VOOR NATUURW ETENSCHAPPEN AARDW ETENSCHAPPEN, 74-SUPPL.: 73-80, 2004

The mode o f life o f Devonian entomozoacean ostracods and the Myodocopid Mega-Assemblage proxy for hypoxic events

by Jean-Georges CASIER

Casier, J.-G., 2004 - The mode o f life o f Devonian entomozoacean ostracods and the M yodocopid M ega-Assem blage proxy for hypoxic events. Bulletin de l ’Institut royal des Sciences naturelles de Belgique, Sciences de la Terre, 74 suppl.: 73-80, 2 text-figs., 1 PL, Bruxelles- Brussel, December 15, 2 0 0 4 - ISSN 0374-6291.

Abstract

The mode o f life o f entomozoacean ostracods, which belong to the M yodocopida, is controversial. It is generally assumed that they are pelagic, more precisely planktonic or nektoplanktonic, and that their presence is indicative o f deep environments. However, a purely mor­phological study is not sufficient to determine their mode o f life. Their relationship with the substrata and other animals, and their response to major environmental changes must be taken into account. These data confirm a swim m ing mode o f life for all the entomozoaceans, and suggest that a nektobenthic lifestyle, in poorly oxygenated, but not necessarily deep, environments is more probable than a planktonic mode o f life. The presence o f a M yodocopid M ega-Assem blage proxy for hypoxic events, seem s more useful that the differentiation o f an “ Entomozoid ecotype” .

K ey-words: Ostracods - Entomozoacea - Devonian - Mode o f life - Palaeoecology.

Résumé

Les Entomozoacea constituent une super-famille d ’ostracodes apparte­nant à l ’ordre des M yodocopida. Ils ont en commun une taille relative­ment grande, des valves égales peu calcifiées, et une ornementation souvent comparée à des empreintes digitales. Elle leur confère une grande valeur biostratigraphique. La plupart des ostracodologistes estiment qu’ils étaient pélagiques et plus précisément planctoniques ou necto-planctoniques, et que leur abondance dans les sédiments témoigne par conséquent d ’une grande profondeur de dépôt. Mais l ’étude morphologique seule ne suffit pas pour déterminer leur mode de vie. Leur relation avec le substrat et avec les autres organismes, ainsi que leur comportement lors des changements environnementaux ma­jeurs doivent aussi être pris en considération. Ils montrent que les Entomozoacea étaient plutôt necto-benthiques et opportunistes dans des m ilieux pauvres en oxygène dissous, mais pas nécessairement profonds. Un M ega-Assem blage à M yodocopida caractérisé par la présence d ’Entomozoacea et (ou) de Cypridinacea (= Assem blage V de C a s ie r et al., 1985; C a s ie r , 2003) peut servir d ’indicateur pour les événem ents hypoxiques dans le Dévonien.

M ots-clefs: Ostracodes - Entomozoacea - Dévonien - Mode de vie - Paléoécologie.

Introduction

Ostracods are ecologically sensitive crustaceans, and their study provides valuable information about environ­mental changes, particularly in the Palaeozoic. For ex­ample, studies o f more than 40,000 ostracods across the Frasnian - Famennian (F/F) boundary from several sec­tions worldwide confirms that the upper Devonian event was one o f the largest Phanerozoic extinctions for ostra­cods. Approximately 75 percent o f all marine ostracod species went extinct close to the F/F boundary ( L e t h i e r s

& C a s i e r , 1999a; C a s i e r & L e t h i e r s , 2001), as a result o f sea-level changes ( C a s i e r & D e v l e e s c h o u w e r , 1995) and a hypoxic event ( C a s i e r , 1987a). The survival o f several species demonstrates that very shallow marine environments and surface waters were not affected during the extinction ( C a s i e r , 2003).

Entomozoacean ostracods played an important role during the Late Devonian mass extinction. However, their mode o f life is controversial. Entomozoaceans are usually regarded as pelagic, more precisely planktonic or nekto­planktonic. The goal o f our paper is to demonstrate that a nektobenthic mode o f life in poorly oxygenated environ­ments is more probable for this group o f ostracods.

The Super-family Entomozoacea P r i b y l , 1951 belongs to the Order M yodocopida S a r s , 1866. Entomozoaceans are characterized by relatively large carapaces with weakly calcified sub-equal valves (Fig. 1 and PI. 1, Figs 9, 10, 12-15 show some species o f entom ozoaceans), the presence in some species o f an adductorial sulcus (= nu­chal furrow), and a ribbed ornam entation frequently com ­pared to fingerprints (see O l e m p s k a , 1992, for a detailed study o f their shell structure). Entom ozoaceans were abundant from the Silurian to the Upper Carboniferous.

Previous statements concerning the mode of life of entomozoaceans

M a t e r n (1929) considered the m ajority o f entom ozoa­ceans to be active swimmers. K u m m e r o w (1939) and R a b i e n (1954) compared entom ozoaceans to m odem Halocypridacea and surmised a nektonic or planktonic

74 Jean-Georges CASIER

Fig. 1 — Richterina (Volkina) zimmermanni ( V o l k , 1939).An example of streamlined entomozoacean in late­ral, dorsal and anterior view. Matagne Formation. Boussu-en-Fagne, Belgium. Collection IRScNB n° a l211. x50.

life habit for this group. On the contrary, the discovery o f thick shelled silicifted carapaces o f entom ozoaceans led B l u m e n s t e n g e l (1965, 1973) to suggest that they were heavily calcified and consequently that they were benthic organisms. B e c k e r (1971) discovered a highly calcified carapace o f entom ozoacean in the Belgian Frasnian, and also concluded that some entomozoaceans displayed a benthic mode o f life. K o z u r (1972) arrived to the con­clusion that they were benthic psychrosphaeric ostracods.

Referring to T r i e b e l (1941), G r ü n d e l , (1962, 1965) and B e c k e r (1976), G o o d a y (1983) summarised the m orphological evidence for a planktonic life habit for entomozoaceans: 1. the presence o f a thin, weakly calci­fied, often streamlined carapace; 2. a convex ventral margin, and 3. the tendency o f the carapace to be rounded with a high surface-volum e ratio. In addition, G o o d a y

(Ibid.) suggested that the presence o f flange-like ribs o f some species may increase the surface area o f the shell and retard its sinking velocity, and that their worldwide distribution, together with their abundance in facies which are often virtually devoid o f other fossils, are supplementary arguments for a planktonic existence. However, G o o d a y (Ibid) noted that the carapace mor­phology o f entom ozoaceans provides no clear evidence for either a planktonic or benthic existence, and he con­cluded that both these life habits may have been repre­sented within the group. He compared some species o f entom ozoaceans with m odem species belonging to the Thaum atocyprididae and considered that there is no con­vincing morphological basis for rejecting the hypothesis that at least some entom ozoaceans were benthic or nek­tobenthic.

After the study o f entom ozoaceans present in the M a­tagne black shale Formation, in Belgium, C a s i e r

(1987a,b, 1988c) arrived to the conclusion that entom o­zoaceans were probably nektobenthic, and adapted to survive in poorly oxygenated, but not necessary deep, environments. However this statement was not supported by G r o o s - U f f e n o r d e & S c h i n d l e r (1990), for whom a nektobenthic mode o f life for entom ozoaceans cannot be assumed because they have not been affected during global events, particularly the Upper Kellwasser Event in the Late Devonian. Their abundance in the Upper Kellwasser Horizon, which according to G r o o s - U f f e ­

n o r d e & S c h i n d l e r (1990) lacks benthos, and their rapid evolution are supplementary arguments against a nekto­benthic lifestyle. Conversely, and also in the Proceedings o f the Tenth International Symposium on Ostracoda, W i l k i n s o n & R i l e y (1990) arrived at the conclusion that entom ozoaceans present in five marine horizons in the Namurian o f Northern England were nektobenthic com ­ponent o f the dysaerobic community. The fauna asso­ciated with the N am urian entom ozoacean in northern England represents low diversity high-abundance assem ­blages typical o f a late Palaeozoic dysaerobic environ­ment, and conclusively W i l k i n s o n & R i l e y (Ibid.) con­sidered the entom ozoacean an integral opportunistic va­grant nektobenthic component o f the dysaerobic assem ­blage, colonizing during b rief spells when the oxygen level was suitably high.

Furthermore in the Proceedings o f the same sym po­sium, B e c k e r & B l e s s (1990) considered C a s i e r

( 1987a,b)’s argum ent that entom ozoaceans may have a nektobenthic, nearshore lifestyle to be inconclusive. They did not observe mixed Eifelian and entom ozoacean as­semblages, and consider this to be evidence that the entom ozoaceans did not live in near-shore environments.

Finally, O l e m p s k a ( 1992) suggested that the extremely fragile thin ornam entation over the whole carapace o f entom ozoaceans is apparently inconsistent with a benthic mode o f life, and that the extremely high flanges may be an adaptation to a planktonic life habit. However, for several species w ith rather heavily calcified shells and relatively short flanges, O l e m p s k a (1992) suggested that an epiplanktonic as well as a planktobenthic life habit is conceivable. It is herein suggested that the presence o f longitudinal canals in the carapace o f some entom ozoa­ceans may have assisted in the exchange o f respiratory gases and such a function could have been important for entom ozoaceans in stagnant and possibly dysaerobic waters.

No argument relevant for a benthic mode o f life for the entom ozoaceans

No particular evidence points conclusively to a genuine benthic mode o f life for the entomozoaceans. Generally, the principal argument for a benthic life habit is the presence o f well calcified and silicifted carapaces in some specimens, which has led some authors to suggest that some or all entom ozoaceans possessed a heavy, thick calcified carapace. The epigenesis in silicon dioxide is

Devonian entomozoacean ostracods and the M yodocopid M ega-Assemblage proxy for hypoxic events 75

frequent during the fossilisation, and there is no doubt that the heavy calcification o f rare carapaces is related to the same phenomenon. The heavy calcified carapace discovered by B e c k e r (1971) in the access path to the Lion quarry, Belgium, is probably an example o f Ento- moprimitia (E.) concentrica ( M a t e r n , 1929), a very abundant species in the Belgian late Frasnian. Such dis­coveries prove only that the carapaces o f entomozoaceans were relatively thick, a feature demonstrated recently by O l e m p s k a (1992). This author confirmed L a n g e r

(1973)’s statement that the carapace o f entomozoaceans consists o f a thin outer layer which formed the ornam en­tation, and a thicker more complicated inner layer. The ornamentation on the upper surface o f the inner layer was generally erroneously interpreted to represent the orna­m entation on the internal surface o f the valve ( O l e m p s k a ,

Ibid.). In other words, the " in te rn a l” moulds o f authors showing the ornamentation corresponds really to exfoli­ate carapaces. It is undoubtable that only the outer layer o f the carapaces o f entomozoaceans were weakly calci­fied, an adaptation to a swimming life habit. The flange­like ribs strengthening the carapace displayed by some species belonging to the genus Richterina confirm this.

One argument for a genuine benthic mode o f life for entomozoaceans, would be the absence o f a rostral in­cisure through which swimming appendages can protrude ( G o o d a y , 1983). R a b i e n (1954) suggested that the ante- ro-ventral curvature o f the carapace observed in some entom ozoaceans might correspond to such a structure, but G o o d a y (1983) demonstrated that the same curvature observed in the recent thaum atocypridids bears no ob­vious relationship to the first and second antennae, the swimming appendages. However, the absence o f a rostral incisure is not an argument for a genuine benthic mode o f life, because that structure lacks in numerous recent swimming ostracods ( G o o d a y , Ibid.).

No argum ent relevant for a nektoplanktonic mode of life for the entomozoaceans

The weakly calcified, sometimes streamlined carapaces e.g.: Richterina (V.) zimmermanni ( V o l k , 1939) (see Fig. 1), occasionally with postero-dorsal spines (possibly used for the balance o f the valves and for the stabilisation o f the carapaces in e.g. some species o f Entomoprimitia and Franklinella), with flange-like ribs (strengthening the carapace and used probably for aerofoils in e.g. R ichte­rina (R.) goodayi O l e m p s k a , 1992), with longitudinal canals (lightening the carapace; see O l e m p s k a , 1992, fig. 8), and with antero-ventral spurs (a structure incom­patible with a benthic mode o f life in the genus Frankli­nella), indicate a swimming life habit for entom ozoa­ceans, but not necessary a nektoplanktonic mode o f life.

A worldwide distribution o f entomozoaceans would be an argument for a nektoplanktonic mode o f life, however this affirm ation is overstated: several species are known from Belgium to the Russian Platform, and North Africa but the relationships with Southern France, China and

North America are limited to rare species, even though all these regions were close during the Late Devonian. Sev­eral indisputable benthic species belonging e.g. to the podocopid ostracods show more widespread geographic distributions compared to that o f entom ozoacean species!

The abundance o f entom ozoaceans in facies which are often virtually devoid o f other fossils is another false argument. Entomozoaceans are always associated with other ostracods or animals. For example, in the Matagne black shale Formation o f Belgium, and in the Cypridie- nen Schiefers o f Germany, entom ozoaceans were always associated with palaeocopid ostracods belonging to the Hollinacea and the Kirkbyacea, and also with bivalves and brachiopods, including lingulids ( S a r t e n a e r , 1974; R a b i e n , 1954). In the more oxygenated environment o f the upper part o f the Valisette Formation, formerly ex­posed in the Senzeille section, Belgium, entomozoaceans are associated with diverse fauna including brachiopods, solitary corals, and Eifelian ecotype ostracods, including podocopids, platycopids, metacopids and palaeocopids ( C a s i e r , 1989). Recently we have also demonstrated that entomozoaceans are mixed with numerous ostracods o f the Eifelian ecotype in the late Frasnian Upper Kellwas­ser Horizon o f the Schmidt quarry parastratotype in the Kellerwald, Germany ( C a s i e r & L e t h i e r s , 1 9 9 8 ) , con­trary to G r o o s - U f f e n o r d e & S c h i n d l e r ( 1 9 9 0 ) ’ s state­ment.

The third argument for a nektoplanktonic mode o f life is the rapid evolution o f the entomozoaceans. Their ap­parently rapid evolution is in reality linked to their dis­tinctive fine ornamentation. Ostracods are generally o f poor biostratigraphic value in the Palaeozoic since criter­ia o f determination are insufficient. They are generally smooth, and due to poor preservation and difficulties o f extraction, internal structures o f carapaces (hinge, muscle scars...) are only very rarely observable. In addition their morphology is highly controlled by environments and convergent species are frequent. For these reasons num er­ous species and genera display very long stratigraphie ranges. For example, the podocopid genus Bairdia Me C o y , 1844, is recognized from the Ordovician to the Recent! In the Devonian, only the thlipsuraceans, with their strong ornamentation, the spiny ostracods o f the Thuringian ecotype o f B e c k e r (in B a n d e l & B e c k e r ,

1975), and the entomozoaceans with their delicate orna­m entation comparable to finger prints, have a biostrati­graphic value. However, without ornamentation, the number o f species distinguishable among the entom ozoa­ceans would be considerably reduced, and consequently their apparent rate o f evolution would be com parable to the rate o f evolution observed for the majority o f other ostracods!

A nektobenthic mode o f life in dysaerobic environ­ments is the most probable for entom ozoaceans

The presence o f homoplasic species in the entom ozoa­ceans (e.g. some Entomoprimitia resemble paraparchita-

76 Jean-Georges CASIER

cean ostracods) is an argum ent in favour o f a relationship with the substrate ( C a s i e r , 1992). O l e m p s k a (1992) ob­served an outermost ridge in some entomozoaceans, an­other interesting example o f convergence. This ridge is comparable to the adventral structure, which continues round the entire valve margin in the palaeocopids, and a nektobenthic life habit is frequently assumed for repre­sentatives o f this group. G r o o s - U f f e n o r d e (1984) ob­served that “ Entom oprim itia” sandbergeri ( M a t e r n ,

1929) displays such a structure, and suggested that the species be excluded from the entom ozoaceans and moved to the palaeocopids!

As mentioned before, in the M atagne black shale For­mation, Belgium, and the Cypridienen Schiefers, G er­many, entom ozoaceans were always associated with pa- laeocopid ostracods belonging to the Flollinacea and the Kirkbyacea, and also with bivalves (Buchiola) and bra­chiopods (Lingulids, Chonetes, Ripidiorhynchus, Ryo- carhynchus etc...), and it is indisputable that these taxa lived in the same environment. The mode o f life o f lingulids is well known because they are still alive, and the life habit o f chonetaceans is easy to imagine because they displayed spines for the fixation on a muddy sub­strate. A nektobenthic mode o f life is assumed for some palaeocopids ( H a r t m a n n , 1963; B e c k e r & B l e s s ,

1990). In fact, all these organisms were adapted to sur­vive in poorly oxygenated environments and this was the case for entomozoaceans. Their weakly calcified cara­pace might be an adaptation to aid survival in poorly oxygenated, calm waters. O l e m p s k a (1992) suggested that the presence o f longitudinal canals observed in the carapace o f some entom ozoaceans may have assisted in exchange o f respiratory gases and such a function could have been important for entom ozoaceans in dysaerobic waters.

In short-term, dysoxic conditions, entomozoaceans were poorly diversified. For instance, in the lower Fras­nian (punctata conodont Zone) observed in the Sourd’Ave section in Belgium, and in the Tempiute M ountain in Nevada, only one species belonging to the genus Franklinella has been recorded ( C a s i e r , 1987b; C a s i e r in S a n d b e r g et al., 1997). However, when such conditions persisted for a long time, for example in the Belgian M atagne Formation or in the German Cypridie­nen Schiefers, entom ozoaceans displayed a gradual di­versification, inducing the developm ent o f large forms, inconsistent with a nektoplanktonic mode o f life.

Our recent study o f ostracods present in the late Fras­nian o f Belgium confirms this life habit ( C a s i e r , 2003). The following assem blages are recognized on the south­ern flank o f the Neuville M assif, w ithin the Dinant Basin ( P I . 1): 1. In the Valisettes Formation enclosing the reef o f the Beauchâteau quarry, the ostracod fauna is largely dom inated by deposit-feeding thick shelled podocopids indicative o f well oxygenated water conditions between fair-weather and storm wave bases; 2. In the upper part o f the Valisettes Formation outcropping in the N euville rail­way section, ostracods belong to the same assemblage (Assemblage III o f C a s i e r , 1987a; see also C a s i e r &

P r e a t , 2003, fig. 3) which is now dom inated by filter- feeding metacopids and palaeocopids. The preponder­ance o f filter-feeding relative to deposit-feeding ostra­cods is considered to signal the start o f an anoxic trend ( W h a t l e y , 1991; L e t h i e r s & W h a t l e y , 1994), and it is most likely related to the increase in water-depth, which, in turn, would have been responsible for the termination o f the reef developm ent as was postulated on basis o f detailed sedimentological analysis ( B o u l v a i n & H e r -

b o s c h , 1996); 3. So extensive was the hypoxia at the Valisettes Formation - M atagne Formation boundary, that ostracods belonging to the benthic Assem blage III dis­appeared abruptly, and were replaced in the black shales o f the overlying M atagne Formation by opportunistic entom ozoaceans and by cypridinaceans (Assem blage V o f C a s i e r , 1987a; see also C a s i e r & P r e a t , 2003, fig. 3 ). These m yodocopid ostracods abruptly disappeared 3.5 m below the Frasnian - Famennian boundary in the Neuville railway section, during the peak o f anoxia, even though Palaeophilomedes neuvillensis C a s i e r , 1988a, an undis- putable nektoplanktonic cypridinacean, survived. The ostracod recovery in the Neuville railway section is m arked by the successive entry o f Franklinella sigm oi­dalis M u l l e r - S t e f f e n , 1964, m á “E ntom oprim itia" sandbergeri ( M a t e r n , 1929). The most important argu­ment for a nektobenthic mode o f life is that the entom o­zoaceans are affected by extinction events, contrary to the suggestion o f G r o o s - U f f e n o r d e & S c h i n d l e r (1990).

Further evidence is provided by the study o f ostracods in the upper Kellwasser Horizon, close to the Frasnian - Famennian boundary, in the Schmidt quarry parastrato- type (Kellerwald, Germany). In this horizon, the relative proportions o f ostracods belonging to the Eifelian eco­type and to the entom ozoaceans reflect variations in the oxygen content o f bottom waters, as suggested by varia­tions o f the total organic carbon contents (TOC) ( C a s i e r

et al., 1999). Entomozaceans undoubtedly becam e extinct during the Late Devonian mass extinction in the Schmidt quarry.

Finally, O l e m p s k a (2002) also displayed that entom o­zoaceans became extinct close to the Frasnian - Fam en­nian boundary in the Plucki section in the Holy Cross Mountains, Poland displaying their persistent anoxic de­position in the linguiformis-triangularis interval ( B o n d et al„ 2004).

The M yodocopid M ega-Assem blage

On the basis o f the carapace morphology, B e c k e r (in B a n d e l & B e c k e r , 1975) differentiated three ostracod ecotypes (= Ökotypen) for the Devonian, including the “ entom ozoid ecotype” . Since then, cypridinacean ostra­cods have been discovered in the Frasnian o f Germany ( B u g g i s c h et a i , 1983; B e c k e r & B l e s s , 1987), o f Belgium ( C a s i e r , 1988a), o f France ( L e t h i e r s & C a s i e r ,

1995), and o f Poland ( O l e m p s k a , 2002) (Fig. 2 and PI. 1, Fig. 11 show a Cypridinacea). Considering that the super­families Cypridinacea and Entomozoacea both belong to

Devonian entomozoacean ostracods and the M yodocopid M ega-Assemblage proxy for hypoxic events 77

Fig. 2 — Palaeophilomedes neuvillensis CASIER, 1988. An example of Cypridinacea. Specimen discovered by T . B e c k e r , in the La Serre coupe C in the Montagne Noire, France. Collection University of Paris 6 n° P6M 1836. xl6.5.

the M yodocopida, and, sharing the opinion o f B e c k e r &

B l e s s (1987) that during the Devonian they lived in the same environments, we suggested it more appropriate to replace the “ entomozoid ecotype” by a “ myodocopid ecotype” ( C a s i e r , et al., 1985). This was an error o f judgm ent. The recent study o f ostracods present in the Neuville railway section ( C a s i e r , 2003) has shown that on the contrary to entomozoaceans, cypridinoid ostracods were not affected during the Late Devonian mass extinc­tion. Moreover, unlike entomozoaceans, cypridinoid os­tracods are sometimes found in environments devoid o f other ostracods and fossils. Consequently we suggest that not all cypridinaceans and entomozoaceans shared the same life strategy during the Devonian: entomozoaceans were nektobenthic and opportunists in poorly oxygenated environments, whereas all or a part o f the cypridinaceans lived in an upper layer o f water, maybe close to the surface. Consequently, and by definition, they were not members o f the same ecotype, and the “ entomozoid ecotype” o f B e c k e r (in B a n d e l & B e c k e r , 1975) re­mains valid but not as emended by the same author in 2001 in order to include the cypridinaceans. In order to distinguish assemblages, a “ M yodocopid M ega-Assem­blage” characterized by the presence o f entomozoaceans and (or) cypridinaceans (= Assemblage V o f C a s i e r et al., 1985; see also fig. 3 in C a s i e r & P r é a t , 2003) is used in this case, rather than an ecotype, since this removes

ambiguity, especially as the usage o f the term ecotype seems to be totally inappropriate in this case. In fact the term ecotype describes a group o f plants, more rarely o f animals, within a single species, and adapted genetically to a particular habitat but able to cross freely with other ecotypes o f the same species ( A b e r c r o m b i e et al., 1980)!

For the same reason, we propose to differentiate an Eifelian M ega-Assemblage constituted o f several assem ­blages (Assemblages 0 to III in C a s i e r , 1987a; see also C a s i e r & P r é a t , 2003, fig. 3) and a Thuringian Mega- Assemblage (= Assemblage V Ibid.).

Conclusions

A purely m orphological study is not sufficient to deter­mine the mode o f life o f entom ozoacean ostracods. Their relations with the substrate and with other animals, and their response to m ajor environmental changes must been taken into account. These data confirm a swimming life habit for the entomozoaceans, and suggest that a nekto­benthic mode o f life in poorly oxygenated environments, is more probable than a planktonic or nektoplanktonic mode o f life for this group.

The distinction o f a “ M yodocopid M ega-Assem ­blage” characterising poorly oxygenated water seems preferable and more useful that the differentiation o f an “ Entom ozoid ecotype” or a “ M yodocopid ecotype” , since the term “ ecotype” seems inappropriate. M ore­over, species belonging to the cypridinaceans and the entomozoaceans did not necessarily share the same en­vironment. This is proved by the fact that cypridinaceans are sometimes found alone (probably during period o f intense anoxia) and by the fact that entomozoaceans becam e extinct during the late Frasnian event whilst cypridinaceans were unaffected by this extinction.

Acknowledgments

Many thanks to Ewa Olempska (Warsaw), and to David Bond (Leeds) for having kindly reviewed this paper. The research has been supported by the F.R.F.C. n° 2.4501.02 project o f the Belgian “ Fonds National de la Recherche Scientifique”

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Jean-Georges C a s ie r

Département de PaléontologieSection de Micropaléontologie-PaléobotaniqueInstitut royal des Sciences naturelles de Belgiqueme Vautier, 29, B-1000 Bruxelles, BelgiqueE-mail: casier@naturalsciences.be

P l a t e 1

Generalized section of the Frasnian - Famennian boundary beds and their ostracod faunas in the type area (Dinant Basin, Belgium). Most representative ostracod species recovered are shown in successive faunas. The succession of assemblages around the Frasnian - Famennian boundary (= FFB) suggests that the Late Devonian mass extinction was principally linked to a regressive-regressive cycle, and to an anoxic event. The transgression was progressive and culminated below the FFB as suspected by J o h n s o n et al. (1985); the regression was more rapid and important. O.M.Z. = oxygen minimum zone. See also comments in the text. More details about these ostracod faunas can be found in: C a s ie r & D e v l e e s c h o u w e r (1995) for ostracods collected in the Sinsin section (Figs. 1 -8 ) ; C a s ie r (2003) for ostracods collected in the Neuville railway section (Figs. 9-22); C a s ie r (1988b) for ostracods collected in the Beauchâteau quarry (Figs. 23-30). The survival of several species and among them of Ovatoquassilites avesnellensis ( L e t h i e r s , 1973) (=Fig. 5) and of the cypridinacean Palaeophilomedes neuvillensis C a s i e r , 1988 (Fig. 11 ) shows that very shallow marine environments and surficial waters were not touched during the Late Devonian mass extinction. Ostracods are deposited in the Department of Palaeontology (section Micropalaeontology) of the Royal Belgian Institute of natural Sciences.

80 Jean-Georges CASIER

HistoricF.F.B.

S E N Z E IL L E

FORM ATION

F.F.B.

8 MATAGNE

Extinction - \ FORM ATION

O.M.Z.

V A L IS E T T E S

FORM ATION

ASS. I very shallow

ASS. V opportu

nistic

ASS. V opportu

nistic

ASS. Ill shallow

P l a t e 1